1. Field of the Invention
The present invention relates to a driving circuit which may be formed as a semiconductor circuit for use in a power IC.
2. Summary of the Prior Art
It is known to form a bridge inverter for driving a load such as a motor by connecting in series two identical voltage-switching elements. Those switching elements are connected across a main d.c. voltage source, with the switching element connected to the high potential side of that source being referred to as an upper arm switching element and the switching element connected to the low potential side of the voltage source being referred to as a lower arm switching element. By switching those switching elements successively, a drive voltage may be generated at their point of interconnection, which drive voltage may be applied to the load.
To drive the upper arm switching element, it is necessary to apply a driving voltage between a control terminal, which controls the switching of the upper arm switching element, and the point of connection between the two switching elements. When the lower arm switching element is switched on, the potential of the point of interconnection between the switching elements drops to a level similar to the low potential side of the main d.c. voltage source. In a similar way, when the upper arm switching element is turned on, the potential of the connection of the two switching elements is similar to that of the high potential side of the main d.c. voltage source.
Hence, in order to drive the upper arm switching element, it is necessary to have a separate d.c. voltage source having a standard mid-point potential between the upper and the lower arms.
Consider now the case where it is desired to generate a polyphase signal to the load. In that case, each phase requires a corresponding pair of upper and lower arm switching elements, and all the lower arms switching elements may be driven from a single power source. However, a separate D.C. insulated power source is required to drive each upper arm switching element, which makes the resulting circuit complex and expensive.
In order to try to resolve this problem, it has been suggested in the Official Gazette of Japanese Patent Laid-Open No. 60-70980 to provide a capacitor and a diode for each phase of the inverter, to formulate a voltage source for driving the upper arm switching elements. The capacitors are charged from a common voltage source.
However, even in this arrangement, one capacitor and one diode must be provided for each phase. Therefore, there is still a limit on the simplicity of the circuit needed. Furthermore, in order to charge the capacitors, it is necessary for the lower arm switching element of the corresponding phase first to have been turned on, so that a current can flow from the power source, through the diode capacitor and lower arm switching element in order for the capacitors to be charged. Therefore, the switching of the upper arm switching element is not independent of the switching of the lower arm switching element and this causes problems at the start-up of the driving circuit since the upper arm switching elements cannot be operated until the lower switching arm elements have been operated. Furthermore, the voltage drop across the capacitor is affected when the lower arm switching element is turned on, and the prior art does not provide any method of compensating for firing the lower arm switching element. Thus, the charging of the capacitor depends on the potential of the interconnection of the two switching elements, which is not satisfactory.
A further disadvantage of the known system is that the driving power to the motor is affected by the power needed to charged the capacitor(s). Again, this is undesirable.